Solve the exponential equation for $x$. 3 x − 9 = 81 5 x + 1 3\^{ x-9}=81\^{5x+1} $x=$
Answer: The strategy We want to rewrite one of the exponential terms in the equation so that the bases of the two terms are the same. Then, we will be able to equate the exponents and solve for $x$. [Why can we do this?] Matching the bases Let's rewrite 81 5 x + 1 81\^{ 5x+1} so its base is $3$. 81 5 x + 1 = ( 3 4 ) 5 x + 1 = 3 4 ( 5 x + 1 ) = 3 20 x + 4 Rewrite 81 as 3 4 Since ( a n ) m = a n ⋅ m \begin{aligned}81\^{5x+1}&= (3^4)\^{ 5x+1} &&&&\text{Rewrite 81 as }3^4\\\\ &=3\^{ 4(5x+1)}&&&&\text{Since }(a^n)^m=a^{n\cdot m} \\\\ &=3\^{ 20x+4}\end{aligned} [Can we choose another base?] Solving the linear equation We obtained the following equation. 3 x − 9 = 3 20 x + 4 3\^{ x-9}=3\^{ 20x+4} Now we can equate the exponents and solve for $x$. $\begin{aligned} x-9 &=20x+4\\\\\\ x &= -\dfrac{13}{19}\end{aligned}$ The answer The answer is $x=-\dfrac{13}{19}$. You can check this answer by substituting $\it{x=-\dfrac{13}{19}}$ in the original equation and evaluating both sides.